Current-injection organic semiconductor laser diode, method for producing same and program

ABSTRACT

Disclosed is a current-injection organic semiconductor laser diode comprising a pair of electrodes, an optical resonator structure, and one or more organic layers including a light amplification layer composed of an organic semiconductor, which has a sufficient overlap between the distribution of exciton density and the electric field intensity distribution of the resonant optical mode during current injection to emit laser light.

TECHNICAL FIELD

The present invention relates to a current-injection organicsemiconductor laser diode and a method for producing it. The presentinvention also relates to a program for designing a current-injectionorganic semiconductor laser diode.

BACKGROUND ART

The properties of optically pumped organic semiconductor lasers (OSLs)have dramatically improved in the last two decades as a result of majoradvances in both the development of high-gain organic semiconductormaterials and the design of high-quality-factor resonator structures¹⁻⁵.The advantages of organic semiconductors as gain media for lasersinclude their high photoluminescence (PL) quantum yields, largestimulated emission cross sections, and broad emission spectra acrossthe visible region along with their chemical tunability and ease ofprocessing. Owing to recent advances in low-threshold distributedfeedback (DFB) OSLs, optical pumping by electrically drivennanosecond-pulsed inorganic light-emitting diodes was demonstrated,providing a route toward a new compact and low-cost visible lasertechnology⁶. However, the ultimate goal is electrically driven organicsemiconductor laser diodes (OSLDs). In addition to enabling the fullintegration of organic photonic and optoelectronic circuits, therealization of OSLDs will open novel applications in spectroscopy,displays, medical devices (such as retina displays, sensors, andphotodynamic therapy devices), and LIFI telecommunications.

The problems that have prevented the realization of lasing by the directelectrical pumping of organic semiconductors are mainly due to theoptical losses from the electrical contacts and the triplet and polaronlosses taking place at high current densities^(4,5,7-9). Approaches thathave been proposed to solve these fundamental loss issues include theuse of triplet quenchers¹⁰⁻¹² to suppress triplet absorption losses andsinglet quenching by singlet-triplet exciton annihilation as well as thereduction of the device active area¹³ to spatially separate whereexciton formation and exciton radiative decay occur and minimize thepolaron quenching processes. However, even with the advances that havebeen made in organic light-emitting diodes (OLEDs) and optically pumpedorganic semiconducting DFB lasers⁵, a current-injection OSLD has stillnot been conclusively demonstrated.

Patent Literature 1^(P1) describes realization of current-injectionOSLD. According to the literature, the device is produced by forming a500 nm-pitch grating (resonator) on an ITO film, then forming a 250nm-thick hole transport layer ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)by vapor deposition, further forming a 100 nm-thick light emitting layerby spin coating with a dichloromethane solution of an aromaticpolycarbonate resin, forming a 250 nm-thick electron transport layer of2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole by vapordeposition, and forming a 200 mm-thick MgAg alloy layer. This literaturedescribes laser emission by voltage application of 30 V to this device.However, in fact, when a dichloromethane solution of a polycarbonate isapplied onto the TPD layer by spin coating, the TPD layer dissolves and,naturally, therefore the device could not be reproduced. In addition,this device has an organic hole transport layer and an organic electrontransport layer each having a thickness of 250 nm formed in addition tothe 100 nm-thick organic light emitting layer therein, and therefore thetotal thickness of the organic layers therein is considerably large. Itis impossible to attain laser oscillation by application of a directcurrent of 30 V to a device having a large total thickness of organiclayers therein.

Other patent literatures^(P2,P3) describe a possibility of realizingcurrent-injection OSLDs. However, these patent literatures merely makegeneral description relating to current-injection OSLDs, not showing atall any specific current-injection OSLD that has confirmed laseroscillation.

PATENT LITERATURES

-   Patent Literature 1: JP-A-2004-186599-   Patent Literature 2: JP-A-H10-321941-   Patent Literature 3: JP-A-2008-524870

Non-Patent Literatures

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SUMMARY OF INVENTION

To that effect, a laser oscillation current-injection OSLD is not yetprovided. An object of the present invention is to provide a laseroscillation current-injection OSLD. As a result of assiduous studies,the present inventors have found that the object can be attained by thepresent invention. The present invention provides a current-injectionorganic semiconductor laser diode comprising a pair of electrodes, anoptical resonator structure, and one or more organic layers including alight amplification layer composed of an organic semiconductor, whichmay satisfy at least one of “2” to “16” below and/or may have at leastone embodiment described below. The present invention includes thefollowing embodiments:

1. A current-injection organic semiconductor laser diode comprising apair of electrodes, an optical resonator structure, and one or moreorganic layers including a light amplification layer composed of anorganic semiconductor, which has a sufficient overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection toemit laser light.

2. The current-injection organic semiconductor laser diode according toitem 1, wherein the optical resonator structure has a distributedfeedback (DFB) structure.

3. The current-injection organic semiconductor laser diode according toitem 2, wherein the optical resonator structure is composed of asecond-order Bragg scattering region surrounded by a first-order Braggscattering region.

4. The current-injection organic semiconductor laser diode according toitem 2, wherein the second-order Bragg scattering region and thefirst-order Bragg scattering region are formed alternately in theoptical resonator structure.

5. The current-injection organic semiconductor laser diode according toany one of items 1 to 4, wherein the number of the one or more organiclayers is 2 or less.

6. The current-injection organic semiconductor laser diode according toany one of items 1 to 5, wherein the thickness of the lightamplification layer relative to the total thickness of the one or moreorganic layers is more than 50%.

7. The current-injection organic semiconductor laser diode according toany one of items 1 to 6, wherein the organic semiconductor contained inthe light amplification layer is amorphous.

8. The current-injection organic semiconductor laser diode according toany one of items 1 to 7, wherein the molecular weight of the organicsemiconductor contained in the light amplification layer is 1000 orless.

9. The current-injection organic semiconductor laser diode according toany one of items 1 to 8, wherein the organic semiconductor contained inthe light amplification layer is a non-polymer.

10. The current-injection organic semiconductor laser diode according toany one of items 1 to 9, wherein the organic semiconductor contained inthe light amplification layer has at least one stilbene unit.

11. The current-injection organic semiconductor laser diode according toany one of items 1 to 10, wherein the organic semiconductor contained inthe light amplification layer has at least one carbazole unit.

12. The current-injection organic semiconductor laser diode according toany one of items 1 to 11, wherein the organic semiconductor contained inthe light amplification layer is 4,4′-bis[(N-carbazole)styryl]biphenyl(BSBCz).

13. The current-injection organic semiconductor laser diode according toany one of items 1 to 12, which has an electron injection layer as oneof the organic layers.

14. The current-injection organic semiconductor laser diode according toitem 13, wherein the electron injection layer contains Cs.

15. The current-injection organic semiconductor laser diode according toany one of items 1 to 14, which has a hole injection layer as aninorganic layer.

16. The current-injection organic semiconductor laser diode according toitem 15, wherein the hole injection layer contains molybdenum oxide.

17. A method for designing a current-injection organic semiconductorlaser diode, comprising:

selecting materials and designing structure of the diode so as toincrease the overlap between the distribution of exciton density and theelectric field intensity distribution of the resonant optical modeduring current injection.

18. A method for producing a current-injection organic semiconductorlaser diode, comprising:

evaluating the overlap between the exciton density distribution and theelectric field intensity distribution of the resonant optical modeduring current injection in a designed or existing diode,

changing at least one of the material and the structure of the diode toso design a new diode as to increase the overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection, and

producing the new diode.

19. A current-injection organic semiconductor laser diode produced bythe method of item 18.

20. A program for designing current-injection organic semiconductorlaser diode, which designs a current-injection organic semiconductorlaser diode to increase the overlap between the distribution of excitondensity and the electric field intensity distribution of the resonantoptical mode during current injection.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1: Organic semiconductor DFB laser diode structure. a, Schematicrepresentation of the organic laser diodes. b, c, Laser microscope (b)and SEM images (c) at 5,000× and 200,000× (inset) magnification of a DFBSiO₂ grating structure prepared on top of ITO. d, Cross-section SEMimages of a complete OSLD. e, Cross-section EDX images of the OSLD. Toimprove visibility of the low-concentration Cs, contrast was enhanced.

FIG. 2: Fabrication and structure of the OSLDs. a, Schematic of thefabrication steps for the OSLDs. b, Structure of the ITO-coated glasssubstrates used in this study along with the general structure of theDFB gratings. Detailed values of the different grating parameters can befound in Table 1. c, d, EDX and SEM analysis of mixed-order DFB gratingprepared on top of ITO. These images confirm the possibility to achieveelectrical contact with ITO.

FIG. 3: Electrical properties of electrically pumped organicsemiconductor DFB laser. a, Energy level diagram of the OSLDs withhighest occupied and lowest unoccupied molecular orbital levelsindicated for the organic and work functions for the inorganics. b,Photomicrographs of an OSLD and a reference OLED under DC operation at3.0 V. The lengths of the individual first- and second-order gratingregions are 1.68 and 1.12 μm. c, d, Current density-voltage (J-V)characteristics (c) and η_(EQE)-J characteristics (d) in the OLED andOSLD under pulsed operation (pulse width of 400 ns and repetition rateof 1 kHz).

FIG. 4: Hole and electron transport in the organic layers. a, b,Architectures of the hole-only device (a) and electron-only device (b)used to evaluate the transport. c, Representative currentdensity-voltage (J-V) characteristics in the hole-only devices (HOD) andelectron-only devices (EOD) under DC operation (filled symbols) andpulse operation (empty symbols) on log and linear (inset) scales. Devicearea is 200×200 μm. These J-V curves indicate good transport of holesand electrons in the high voltage region in the laser diodes fabricatedin this study. Current at low voltages is higher for electrons thanholes because of trap limiting of the hole current.

FIG. 5: Properties of OSLDs with different DFB geometries. a,Photomicrographs of OSLDs under DC operation at 3.0 V. Thephotomicrographs were take using the same magnification, and all gratingrun vertically. b, c, d, Current density-voltage (J-V) and η_(EQE)-Jcharacteristics of the OSLDs. e, Electroluminescence intensity and FWHMas a function of J. f, Emission spectra collected in the directionnormal to the substrate plane as a function of J.

FIG. 6: Direct-current characteristics and emission spectra of devices.a, b, Current density-voltage (J-V) curves (a) and η_(EQE)-J curves (b)of the OLED and OSLD measured under DC operation. c, PL spectra of aneat BSBCz film (black line) and EL spectra of the OLED (red line) andOSLD below the lasing threshold (blue line).

FIG. 7: Lasing properties of OSLDs. a, Emission spectra of an OSLDcollected in the direction normal to the substrate plane for differentinjected current densities. At current densities higher than 3.5 kAcm⁻², serious device degradation at the lasing wavelength results in astrong increase in the background EL relative to the lasing. b, Emissionspectra near the lasing threshold. c, Output intensity and FWHM as afunction of the current. d, Output power as a function of the current.The inset is a photograph of an OSLD under pulsed operation at 50 V.

FIG. 8: Characterization of the emission from OSLDs. a, Emission spectraof an OSLD above threshold measured at different polarization angles.Polarization is stronger above threshold (inset, circles) than belowthreshold (inset, triangles). Here, 90° corresponds to the directionparallel to the grooves of the DFB grating. b, c, CCD camera images (b)and cross-sections (c) showing the spatial Gaussian profiles of afocused emission beam from an OSLD at different current densities. d, e,Unfocused beam of an OSLD operating above threshold projected on ascreen.

FIG. 9: Characteristics of OSLD under optical pumping. a, Test set-upused to measure the beam profile in FIG. 8b, c . b, Characteristics andnear-field beam images and cross-sections of an OSL (see Table 1 forstructure) under optical excitation below threshold (i), near threshold(ii), and above threshold (iii). c, Characteristics and near-field beamimages and cross-sections of an OSLD-6 (see Table 1 for structure) underoptical excitation below threshold (iv), near threshold (v), and abovethreshold (v). d, Far-field beam cross-sections of an OSL under opticalexcitation above threshold, near threshold, and below threshold alongwith the simulated far-field beam above threshold. The inset for abovethreshold is the original emission pattern. e, Emission spectracollected in the direction normal to the substrate plane for OSLD-6under optical pumping with different photoexcitation densities. Thesteady-state photoluminescence spectra of BSBCz on SiO₂ with a gratingis shown as a dashed line. f, Output intensity and FWHM of OSLD-6 as afunction of the photoexcitation density. Excitation was for 3.0 ns by aN₂ laser at 337 nm, and the device was at ambient temperature. g, h, i,Slope efficiency of an optically pumped OSL (g, see Table 1 forstructure), an electrically pumped OSLD (h), and an optically pumpedOSLD-6 (i). Input power for the optically pumped devices is the power ofthe source and was incident on the organic film side for the OSL and theglass side for the OSLD-6.

FIG. 10: Absorption spectra of triplets and radical cations and anionsof BSBCz. a, Stimulated emission and triplet absorption cross-sectionspectra of BSBCz. Emission spectra of the OSL were measured from a BSBCzneat film above threshold. b, To investigate the spectral overlapbetween the components, the absorption spectra of neat films of BSBCz(50 nm, black) and composite films BSBCz:MoO₃ and BSBCz:Cs, (1:1 molratio, 50 nm; blue and red, respectively) were measured. The absorptionspectra were measured using an absorption spectrometer (Lamda 950,PerkinElmer). The steady-state PL spectrum of a BSBCz neat film (green)and a representative laser emission spectrum (orange) from an OSL underoptical pumping are also displayed to show that polaron absorption inBSBCz OSLDs should be negligible.

FIG. 11: Optical and electrical simulations. a, Experimental (symbol)and simulated (solid line) J-V curves for hole-only device (bluecircles), electron-only device (red squares), and bipolar device (blacktriangles). Model parameters were extracted by fitting to the unipolardevices from FIG. 4, and those parameters were used for simulating thebipolar device. b, Comparison of mobilities calculated using theparameters extracted from the unipolar devices (solid lines) withreported⁴¹ mobilities (symbols) for holes (blue) and electrons (red) inBSBCz. c, Experimental (symbols) and simulated (solid line) J-V curvesfor the OSLD. d, Schematic of the OSLD structure used for thecalculations. e, Spatial distribution of the recombination rate profile,R, for the OSLD at J=500 mA cm⁻². f, Cross section through (e) at y=0.11μm for the DFB device. g, Average exciton density as a function of thecurrent density for the OSLD and OLED.

FIG. 12: Simulations of the OSLD. a, Spatial distribution of the excitondensity, S. b, Electric field distribution of the passive DFB resonantcavity at the resonant wavelength λ₀=483 nm for a structure extended toinclude first-order regions. c, Modal gain as a function of currentdensity. d, Spatial overlap between the exciton density S(x, y) and theoptical mode |E(x, y)|² for one period in the second-order region atJ=500 A cm⁻². Layers other than the grating were modeled as being flat(see FIG. 11d ), and y=0 corresponds to the BSBCz/MoO₃ interface.

DETAILED DESCRIPTION OF INVENTION

The contents of the invention will be described in detail below. Theconstitutional elements may be described below with reference torepresentative embodiments and specific examples of the invention, butthe invention is not limited to the embodiments and the examples. In thepresent specification, a numerical range expressed by “from X to Y”means a range including the numerals X and Y as the lower limit and theupper limit, respectively.

All the literatures and the description of PCT/JP2017/033366 as referredto herein are incorporated herein by reference.

The current-injection OSLD of the present invention contains at least apair of electrodes, an optical resonator structure and one or moreorganic layers containing a light amplification layer composed of anorganic semiconductor. The current-injection OSLD of the presentinvention is provided with a constitution where the overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection issufficient to emit laser light. The “constitution where the overlapbetween the distribution of exciton density and the electric fieldintensity distribution of the resonant optical mode during currentinjection is sufficient to emit laser light” is a constitution to enablelaser oscillation, and means selection and combination of materials andstructures to be described below.

The constitution and the characteristics of the present invention aredescribed in detail hereinunder.

(Light Amplification Layer)

The light amplification layer to constitute the current-injection OSLDof the present invention includes an organic semiconductor compoundcontaining a carbon atom but not containing a metal atom. Preferably,the organic semiconductor compound is composed of one or more atomsselected from the group consisting of a carbon atom, a hydrogen atom, anitrogen atom, an oxygen atom, a sulfur atom, a phosphorous atom, and aboron atom. For example, there may be mentioned an organic semiconductorcompound composed of a carbon atom, a hydrogen atom and a nitrogen atom.A preferred example of the organic semiconductor compound is a compoundhaving at least one of a stilbene unit and a carbazole unit, and a morepreferred example of the organic semiconductor compound is a compoundhaving both of a stilbene unit and a carbazole unit. The stilbene unitand the carbazole unit may be substituted with a substituent such as analkyl group or the like, or may be unsubstituted. Preferably, theorganic semiconductor compound is a non-polymer not having a repeatingunit. Preferably, the molecular weight of the compound is 1000 or less,for example, it may be 750 or less. The light amplification layer maycontain 2 or more kinds of organic semiconductor compounds, butpreferably contains only one kind of an organic semiconductor compound.

The organic semiconductor compound for use in the present invention maybe selected from laser gain organic semiconductor compounds that enablelaser oscillation when used in an organic light emitting layer of aphotoexcitation organic semiconductor laser. One of the most preferableorganic semiconductor compound is 4,4′-bis[(N-carbazole)styryl]biphenyl(BSBCz) (chemical structure in FIG. 1a )¹⁵ because of its excellentcombination of optical and electrical properties such as a low amplifiedspontaneous emission (ASE) threshold in thin films (0.30 μJ cm⁻² under800-ps pulse photoexcitation)¹⁶ and the ability to withstand theinjection of current densities as high as 2.8 kA cm⁻² under 5-μs pulseoperation in OLEDs with maximum electroluminescence (EL) externalquantum efficiencies (η_(EQE)) of over 2%¹³. Furthermore, lasing at ahigh repetition rate of 80 MHz and under long pulse photoexcitation of30 ms were recently demonstrated in optically pumped BSBCz-based DFBlasers¹⁷ and were largely possible because of the extremely smalltriplet absorption losses at the lasing wavelength of BSBCz films. Apartfrom BSBCz, also employable are, for example, compounds having an ASEthreshold of preferably 0.60 μJ cm⁻² or less, more preferably 0.50 μJcm⁻² or less, even more preferably 0.40 μJ cm⁻² or less, when formedinto the same thin film as in the literature 16 and measured under the800-ps pulse photoexcitation condition. In addition, compounds areemployable, which exhibit durability of preferably 1.5 kA cm⁻² or more,more preferably 2.0 kA cm⁻² or more, even more preferably 2.5 kA cm⁻² ormore, when formed into the same device as in the literature 13 andmeasured under the 5-μs pulse operation condition.

The thickness of the light amplification layer to constitute thecurrent-injection OSLD of the present invention is preferably 80 to 350nm, more preferably 100 to 300 nm, even more preferably 150 to 250 nm.

(Other Layers)

The current-injection OSLD of the present invention may have an electroninjection layer, a hole injection layer and others in addition to thelight amplification layer. These may be organic layers or inorganiclayers free from organic materials. In the case where thecurrent-injection OSLD has two or more organic layers, it preferably hasa laminate structure of organic layers alone not having any non-organiclayer therebetween. In this case, the two or more organic layers maycontain the same organic compound as in the light amplification layer.The performance of the current-injection OSLD tends to be better whenthe number of the heterointerfaces of the organic layers therein issmaller, and therefore, the number of the organic layers therein ispreferably 3 or less, more preferably 2 or less, most preferably 1. Inthe case where the current-injection OSLD has 2 or more organic layers,preferably, the thickness of the light amplification layer is more than50% of the total thickness of the organic layers, more preferably morethan 60%, even more preferably more than 70%. When the current-injectionOSLD has 2 or more organic layers, the total thickness of the organiclayers may be, for example, 100 nm or more, 120 nm or more, or 170 ormore, and may be 370 nm or less, 320 nm or less, or 270 nm or less.Preferably, the refractive index of the electron injection layer and thehole injection layer is smaller than the refractive index of the lightamplification layer.

In the case where an electron injection layer is provided, a substancefacilitating electron injection into the light amplification layer ismade to exist in the electron injection layer. In the case where a holeinjection layer is provided, a substance facilitating hole injectioninto the light amplification layer is made to exist in the holeinjection layer. These substances may be an organic compound or aninorganic substance. For example, the inorganic substance for theelectron injection layer includes an alkali metal such as Cs, etc., andthe concentration thereof in the electron injection layer containing anorganic compound may be, for example, 1% by weight more, or 5% by weightor more, or 10% by weight or more, and may be 40% by weight or less, or30% by weight or less. The thickness of the electron injection layer maybe, for example, 3 nm or more, 10 nm or more, or 30 nm or more, and maybe 100 nm or less, 80 nm or less, or 60 nm or less.

As one preferred embodiment of the present invention, acurrent-injection OSLD provided with an electron injection layer and alight amplification layer as organic layers, and with a hole injectionlayer as an inorganic layer may be exemplified. The inorganic substanceto constitute the hole injection layer includes a metal oxide such asmolybdenum oxide, etc. The thickness of the hole injection layer may be,for example, 1 nm or more, 5 nm or more, or 10 nm or more, and may be100 nm or less, 50 nm or less, or 20 nm or less.

(Optical Resonator Structure)

The current-injection OSLD of the present invention has an opticalresonator structure. The optical resonator structure may be aone-dimensional resonator structure or a two-dimensional resonatorstructure. Examples of the latter include a circulator resonatorstructure, and a whispering gallery type optical resonator structure. Adistributed feedback (DFB) structure and a distributed Bragg reflector(DBR) structure are also employable. For DFB, a mixed-order DFB gratingstructure is preferably employed. Namely, a mixed structure of DFBgrating structures differing in point of the order relative to laseremission wavelength may be preferably employed. Specific examplesthereof include an optical resonator structure composed of asecond-order Bragg scattering region surrounded by the first-order Braggscattering region and a mixed structure where a second-order Braggscattering region and a first-order scattering region are formedalternately. For details of preferred optical resonator structures,specific examples to be given hereinunder may be referred to. As theoptical resonator structure, the current-injection OSLD may be furtherprovided with an external optical resonator structure.

For example, the optical resonator structure may be formed preferably onan electrode. The material to constitute the optical resonator structureincludes an insulating material such as SiO₂, etc. For example, agrating structure is formed, the depth of the grating is preferably 75nm or less, and is more preferably selected from a range of 10 to 75 nm.The depth may be, for example, 40 nm or more, or may be less than 40 nm.

(Electrode)

The current-injection OSLD of the present invention has a pair ofelectrodes. For light output, one electrode is preferably transparent.For the electrode, an electrode material generally used in the art maybe appropriately selected in consideration of the work function thereof,etc. Preferred electrode materials include, though not limited thereto,Ag, Al, Au, Cu, ITO, etc.

(Preferred Current-Injection OSLD) In the current-injection OSLD,excitons are generated by current excitation. The laser oscillationcharacteristics of the current-injection OSLD is improved by enlargingthe overlap between the distribution region of the generated excitondensity and the electric field intensity distribution of the resonantoptical mode. Namely, when the exciton density overlaps with the opticalresonant mode of the optical resonator structure, the laser oscillationcharacteristics can be thereby improved. The exciton densitydistribution and the electric field intensity distribution of theresonant optical mode can be controlled by changing the structure andthe material of the current-injection OSLD. For example, by employing acurrent narrowing structure with grating or the like, and by controllingthe depth and the period of the grating, the distributions can becontrolled. By specifying or controlling the materials and the thicknessof the light amplification layer, and also the materials and thethickness of an electron injection layer and a hole injection layer, ifany, the distributions can also be controlled. Further, a more accuratedistribution control is also possible in view of the conditions takeninto consideration in the simulation calculation to be given below.Preferable current-injection OSLD has an overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection to anextent equal to or more than the overlap in the specificcurrent-injection OSLD shown as a working example below.

In the current-injection OSLD of the present invention, preferably, theratio of an electron mobility to a hole mobility in the organic opticalgain layer is controlled to fall within a range of preferably 1/10 to10/1, more preferably 1/5 to 5/1, still more preferably 1/3 to 3/1,still further more preferably 1/2 to 2/1. By controlling the ratio tofall within the range, the overlap between the exciton densitydistribution and the electric field intensity distribution of theresonant optical mode can be readily enlarged.

In the current-injection OSLD of the present invention, preferably, theexcitons generated by current excitation do not substantially undergoannihilation. The loss by the exciton annihilation is preferably lessthan 10%, more preferably less than 5%, further more preferably lessthan 1%, still further more preferably less than 0.1%, still furthermore preferably less than 0.01%, and most preferably 0%.

Also preferably, the current-injection OSLD of the present inventionshows no substantial polaron absorption loss at a lasing wavelength. Inother words, preferably, there is no substantial overlap between thepolaron absorption spectrum and the emission spectrum of the organicsemiconductor laser. The loss by the polaron absorption is preferablyless than 10%, more preferably less than 5%, further more preferablyless than 1%, still further more preferably less than 0.1%, stillfurther more preferably less than 0.01%, and most preferably 0%.

Preferably, the oscillation wavelength of the current-injection OSLD ofthe present invention does not substantially overlap with the absorptionwavelength region of an excited state, a radical cation, or a radicalanion. Absorption in them may be caused by singlet-singlet,triplet-triplet, or polaron absorption. The loss by absorption in anexcited state is preferably less than 10%, more preferably less than 5%,further more preferably less than 1%, still further more preferably lessthan 0.1%, still further more preferably less than 0.01%, and mostpreferably 0%.

Preferably, the current-injection OSLD of the present invention is freefrom a triplet quencher.

(Production Method for Current-Injection OSLD)

The present invention also provides a production method for thecurrent-injection OSLD where the current-injection OSLD is designed andproduced so that the overlap between the distribution of the excitondensity generated by current excitation and the electric field intensitydistribution of the resonant optical mode can be large. In designing it,simulation is carried out on the basis of various conditions, forexample, the depth and the period of grating, and the constituentmaterial and the thickness of the light amplification layer, theelectron injection layer and the hole injection layer, etc., and theoverlap between the exciton density distribution and the electric fieldintensity distribution of the resonant optical mode is therebyevaluated. As a result of simulation under various conditions, ones withno problem in production are selected from those where the overlap isevaluated to be large, and the thus-selected ones may be actuallyproduced. Accordingly, current-injection OSLDs having excellent laseroscillation characteristics can be efficiently provided.

In designing the above, a design program for current-injection OSLDshaving a function of so designing them in such a manner that the overlapbetween the distribution of the excitons to be generated by currentexcitation and the electric field intensity distribution of the resonantoptical mode can be enlarged may be previously formed and used. Theprogram can be stored in a media such as a hard disc and a compact disc.

Also, the present invention provides a method for improving the laseroscillation characteristics of designed or existing current-injectionOSLDs. The overlap between the exciton density distribution and theelectric field intensity distribution of the resonant optical mode indesigned or existing current-injection OSLDs is evaluated according tosimulation calculation, and the distribution overlap in the case wherethe materials and the structure have been changed is also calculatedthrough the same simulation calculation, and accordingly,current-injection OSLDs that have improved laser oscillationcharacteristics can be thereby provided.

Preferred Embodiments of Invention

Hereinunder the present invention will be described concretely withreference to the preferred embodiment shown in FIG. 1a . However, thescope of the present invention should not be limitatively interpreted bythe following concrete description.

The properties of optically pumped organic semiconductor lasers (OSLs)have dramatically improved in the last two decades as a result of majoradvances in both the development of high-gain organic semiconductormaterials and the design of high-quality-factor resonator structures¹⁻⁵.The advantages of organic semiconductors as gain media for lasersinclude their high photoluminescence (PL) quantum yields, largestimulated emission cross sections, and broad emission spectra acrossthe visible region along with their chemical tunability and ease ofprocessing. Owing to recent advances in low-threshold distributedfeedback (DFB) OSLs, optical pumping by electrically drivennanosecond-pulsed inorganic light-emitting diodes was demonstrated,providing a route toward a new compact and low-cost visible lasertechnology⁶. However, the ultimate goal is electrically driven organicsemiconductor laser diodes (OSLDs). In addition to enabling the fullintegration of organic photonic and optoelectronic circuits, therealization of OSLDs will open novel applications in spectroscopy,displays, medical devices (such as retina displays, sensors, andphotodynamic therapy devices), and LIFI telecommunications.

The problems that have prevented the realization of lasing by the directelectrical pumping of organic semiconductors are mainly due to theoptical losses from the electrical contacts and the triplet and polaronlosses taking place at high current densities^(4,5,7-9). Approaches thathave been proposed to solve these fundamental loss issues include theuse of triplet quenchers¹⁰⁻¹² to suppress triplet absorption losses andsinglet quenching by singlet-triplet exciton annihilation as well as thereduction of the device active area¹³ to spatially separate whereexciton formation and exciton radiative decay occur and minimize thepolaron quenching processes. However, even with the advances that havebeen made in organic light-emitting diodes (OLEDs) and optically pumpedorganic semiconducting DFB lasers⁵, a current-injection OSLD has stillnot been conclusively demonstrated.

Previous studies have suggested that current densities over a few kA/cm²would be required to achieve lasing from an OSLD if additional lossesassociated with the electrical pumping were completely suppressed¹⁴. Oneof the most promising molecules for the realization of an OSLD is4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) (chemical structure inFIG. 1a )¹⁵ because of its excellent combination of optical andelectrical properties such as a low amplified spontaneous emission (ASE)threshold in thin films (0.30 μJ cm⁻² under 800-ps pulsephotoexcitation)¹⁶ and the ability to withstand the injection of currentdensities as high as 2.8 kA cm⁻² under 5-μs pulse operation in OLEDswith maximum electroluminescence (EL) external quantum efficiencies(η_(EQE)) of over 2%¹³. Furthermore, lasing at a high repetition rate of80 MHz and under long pulse photoexcitation of 30 ms were recentlydemonstrated in optically pumped BSBCz-based DFB lasers¹⁷ and werelargely possible because of the extremely small triplet absorptionlosses at the lasing wavelength of BSBCz films. Here, the inventorsundoubtedly demonstrate the first examples of lasing from an organicsemiconductor film directly excited by electricity through thedevelopment and complete characterization of OSLDs based on a BSBCz thinfilm in an inverted OLED structure with a mixed-order DFB SiO₂ gratingintegrated into the active area of the device.

The architecture and fabrication of the OSLDs developed in this studyare schematically represented in FIG. 1a and in FIG. 2 (see theMaterials and Methods for a detailed description of the experimentalprocedures). A sputtered layer of SiO₂ on indium tin oxide (ITO) glasssubstrates was engraved with electron beam lithography and reactive ionetching to create mixed-order DFB gratings with an area of 30×90 μm(FIG. 1b ), and organic layers and a metallic cathode were vacuumdeposited on the substrates to complete the devices. The inventorsdesigned the mixed-order DFB gratings to have first- and second-orderBragg scattering regions that provide strong optical feedback andefficient vertical outcoupling of the laser emission,respectively^(17,18.) Grating periods (Λ₁ and Λ₂) of 140 and 280 nm werechosen for the first- and second-order regions, respectively, based onthe Bragg condition^(4,19), mλ_(Bragg)=2n_(eff)Λ_(m), where m is theorder of diffraction, λ_(Bragg) is the Bragg wavelength, which was setto the reported maximum gain wavelength (477 nm) for BSBCz, and n_(eff)is the effective refractive index of the gain medium, which wascalculated to be 1.70 for BSBCz^(20,21). The lengths of the individualfirst- and second-order DFB grating regions were 1.12 and 1.68 μm,respectively, in the first set of devices characterized, hereafterreferred to as OSLDs.

The scanning electron microscopy (SEM) images in FIGS. 1c and d confirmthat the fabricated DFB gratings had periods of 140±5 and 280±5 nm witha grating depth of about 65±5 nm. Complete removal of the SiO₂ layer inthe etched areas to expose the ITO is important for making goodelectrical contact with the organic layer and was verified with energydispersive X-ray spectroscopy (EDX) analysis (FIG. 2c, d ).Cross-sectional SEM and EDX images of a complete OSLD are shown in FIGS.1d and e . The surface morphologies of all layers present a gratingstructure with a surface modulation depth of 50-60 nm. Although theinteraction of the resonating laser mode with the electrodes is expectedto reduce the quality factor of the feedback structure, such a gratingstructure on the metal electrode should also reduce the absorptionlosses of a mode guided within the device structure^(22,23).

The OSLDs fabricated in this work have a simple inverted OLED structureof ITO (100 nm)/20 wt. % Cs:BSBCz (60 nm)/BSBCz (150 nm)/MoO₃ (10 nm)/Ag(10 nm)/Al (90 nm) with the energy levels as shown in FIG. 3a (workingexample). Doping the BSBCz film with Cs in the region close to the ITOcontact improves the electron injection into the organic layer, and MoO₃is used as a hole injection layer (FIG. 4). While the most efficientOLEDs generally use multilayer architectures to optimize chargebalance^(24,25), charges can accumulate at organic hetero-interfaces athigh current densities, which can be detrimental for device performanceand stability²⁶. The OSLDs fabricated in this work contained only BSBCzas the organic semiconductor layer (light amplification layer) and werespecifically designed to minimize the number of organichetero-interfaces. Reference devices without an SiO₂ DFB grating,hereafter referred to as OLEDs, were also fabricated to investigate theinfluence of the gratings on the EL properties.

FIG. 3b shows optical microscope images of an OSLD and a reference OLEDunder direct current (DC) operation at 3.0 V. In addition to thepreviously described DFB grating, five other DFB grating geometries(Table 1) were optimized and characterized in OSLDs. While the EL isemitted homogeneously from the active area of the reference OLEDs, moreintense emission can be seen from the second-order DFB grating regions,which were specifically designed to promote vertical light outcoupling,in the OSLDs (FIG. 3b and FIG. 5). The current density-voltage (J-V) andη_(EQE)-J characteristics in an OSLD and OLED under pulsed conditions(voltage pulse width of 400 ns and repetition rate of 1 kHz) at ambienttemperatures are shown in FIGS. 2c and d , and the characteristicsobtained under DC conditions are displayed in FIG. 6. Though somecurrent flows through the areas above the SiO₂ grating (˜20% based onsimulations), most flows through the areas above the exposed ITO. Forsimplicity and consistency, the exposed ITO area was used for thecalculation of current density for all OSLDs, though this may lead toslight overestimations.

TABLE 1 Parameters for the different OSLD geometries Device w (μm) l(μm) Λ₁ (nm) Λ₂ (nm) w₁ (μm) w₂ (μm) A (μm²) OLED 30 45 — — — — 1,350OSLD 30 90 140 280 1.68 1.12 1,350 OSLD-1 35 90 140 280 14.00 7.00 1,575OSLD-2 90 30 140 280 1.68 1.12 1,350 OSLD-3 101 30 140 280 45.36 10.081,515 OSLD-4 30 90 134 268 1.608 1.072 1,350 OSLD-5 30 90 146 292 1.7521.168 1,350 OSLD-6 560 800 140 280 1.68 1.12 224,000  OSL 5,000 5,000140 280 15.12 10.08 —Values of the different grating geometries for the parameters shown inFIG. 2 along with the total exposed ITO area A used for calculatingcurrent density. The OSL is a 200-nm-thick layer of BSBCz deposited on agrating on a fused silica substrate and does not included contacts.

The maximum current densities before device breakdown of the referenceOLEDs increased from 6.6 A cm⁻² under DC operation to 5.7 kA cm⁻² underpulse operation because of reduced Joule heating with pulseoperation^(13,27). Under DC operation, all of the devices exhibitedmaximum η_(EQE) higher than 2% at low current densities and strongefficiency rolloff at current densities higher than 1 A cm⁻², which ispresumably due to thermal degradation of the devices. On the other hand,efficiency rolloff in the OLEDs under pulse operation (FIG. 3c, d )began at current densities higher than 110 A cm⁻², consistent with aprevious report¹³. Efficiency rolloff was further suppressed in theOSLDs under pulse operation, and the η_(EQE) was even found tosubstantially increase above 200 A cm⁻² to reach a maximum value of2.9%. The rapid decrease in η_(EQE) above a current density of 2.2 kAcm⁻² is likely due to thermal degradation of the device.

While the EL spectra of the OLEDs are similar to the steady-state PLspectrum of a neat BSBCz film (FIG. 6c ) and did not change as afunction of the current density, the EL spectra from the glass face ofthe OSLDs under pulse operation exhibited spectral line narrowing withincreasing current density (FIG. 7a ). A Bragg dip corresponding to thestopband of the DFB grating was observed at 478.0 nm for currentdensities below 650 A cm⁻² (FIG. 7b ). As the current density increasedabove this value, strong spectral line narrowing occurs at 480.3 nm,suggesting the onset of lasing. The intensity of the narrow emissionpeak was found to increase faster than that of the EL emissionbackground, which could be attributed to the non-linearity associatedwith stimulated emission.

The output intensity and full-width-at-half-maximum (FWHM) of an OSLDare plotted in FIG. 7c as a function of the current. While the FWHM ofthe steady-state PL spectrum of a neat BSBCz film is around 35 nm, theFWHM of the OSLD at high current densities decreases to values lowerthan 0.2 nm, which is close to the spectral resolution limit of thespectrometer used in the invention (0.17 nm for a wavelength range of 57nm). The slope efficiency of the output intensity abruptly changes withincreasing current and can be used to determine a threshold of 600 Acm⁻² (8.1 mA). Above 4.0 kA cm⁻², the output intensity decreases withincreasing current, presumably because of a strong increase oftemperature leading to the onset of device breakdown, but the emissionspectrum remains extremely sharp. This increase and subsequent decreaseis consistent with the η_(EQE)-J curve. The maximum output powermeasured with a power meter placed in front of an OSLD at a distance of3 cm away from the ITO glass substrate (FIG. 7d ) was 0.50 mW at 3.3 kAcm⁻². These observed EL properties strongly suggest that lightamplification occurs at high current densities and that electricallydriven lasing is achieved above a current density threshold.

Beam polarization and shape were characterized to provide furtherevidence that this is lasing⁹. The output beam of an OSLD is stronglylinearly polarized along the grating pattern (FIG. 8a ), which isexpected for laser emission from a one-dimensional DFB. Spatial profilesof focused OSLD emission (FIG. 8b and FIG. 9a ) show the existence ofwell-defined Gaussian beams with a diameter of around 0.1 mm (FIG. 8c ),demonstrating the excellent focusability of the output beam from theOSLD above the lasing threshold. Projection of the beam on a screenresults in a fan-shaped pattern as expected for a one-dimensional DFB(FIG. 8d, e ). Rapid degradation above threshold preventedinterferometry measurements at this stage, but the inventors estimatedcoherence lengths (L) from the equation L=λ_(peak) ²/FWHM, whereλ_(peak) is the peak wavelength, of 1.1-1.3 mm for all of the devices inthis report, which is also consistent with lasing. Under opticalexcitation, which results in slower degradation, near-field beampatterns of similar device structures with and without the electrodes(FIG. 9b, c ) are similar, further indicating that the devices cansupport lasing. Additionally, the far-field patterns under opticalexcitation are also consistent with lasing (FIG. 9d ).

Before the inventors can claim lasing, several phenomena that have beenmisinterpreted as lasing in the past must be ruled out as the cause ofthe observed behavior⁹. The emission from the inventors' OSLDs isdetected in the direction normal to the substrate plane and shows aclear threshold behavior, so line narrowing arising from edge emissionof waveguided modes without laser amplification can bedismissed^(20,28,29). ASE can appear similar to lasing, but the FWHM inOSLDs of the invention (<0.2 nm) is much narrower than the typical ASElinewidth of an organic thin film (a few nanometers) and is consistentwith the typical FWHM of an optically pumped organic DFB lasers (<1nm)⁵. A very narrow emission spectrum obtained by inadvertently excitingan atomic transition in ITO has also been mistaken for emission from anorganic layer³⁰. However, the emission peak wavelength of the OSLD inFIG. 7a is 480.3 nm and cannot be attributed to emission from ITO, whichhas atomic spectral lines at 410.3, 451.3, and 468.5 nm.³¹

If this truly is lasing from a DFB structure, then the emission of theOSLD should be characteristic of the resonator modes and the outputshould be very sensitive to any modifications of the laser cavity. Thus,OSLDs with different DFB geometries, labeled OSLD-1 through OSLD-5(Table 1), were fabricated and characterized (FIG. 5) to confirm thatthe emission wavelength could be predictably tuned, which is common inoptically pumped organic DFB lasers^(4,5,32,33). The lasing peaks arenearly the same for OSLD, OSLD-1, OSLD-2, and OSLD-3 (480.3 nm, 479.6nm, 480.5 nm, and 478.5 nm, respectively), which have the same DFBgrating periods. Furthermore, OSLD-1, OSLD-2, and OSLD-3 all had lowminimum FWHM (0.20 nm, 0.20 nm, and 0.21 nm, respectively) and clearthresholds (1.2 kA cm⁻², 0.8 kA cm⁻², and 1.1 kA cm⁻², respectively). Onthe other hand, OSLD-4 and OSLD-5, which have different DFB gratingperiods, exhibited lasing peaks at 459.0 nm with a FWHM of 0.25 nm and athreshold of 1.2 kA cm⁻² (OSLD-4) and 501.7 nm with a FWHM of 0.38 nmand a threshold of 1.4 kA cm⁻² (OSLD-5). These results clearlydemonstrate that the lasing wavelength is being controlled by the DFBgeometry.

To verify that the lasing threshold of the electrically driven OSLD isconsistent with that obtained by optical pumping, the lasingcharacteristics of an OSLD (OLSD-6) optically pumped through the ITOside using a N₂ laser (excitation wavelength of 337 nm) delivering3.0-ns pulses were measured (FIG. 9e, f ). The lasing peak of OLSD-6under optical pumping (481 nm) is consistent with that of the OSLDsunder electrical pumping (480.3 nm). The lasing threshold under opticalpumping was measured to be around 77 W cm⁻² when considering only thepower coupled into the device (˜18% based on simulations). Therelatively small increase in threshold compared to that obtained inoptically pumped BSBCz-based DFB lasers without the two electrodes (30 Wcm⁻²)¹⁷ is a result of optimization of layer thickness to minimize theoptical losses arising from the presence of the electrodes. Assuming noadditional loss mechanisms in OSLD-6 at high current densities, a lasingthreshold of 0.3 kA cm⁻² under electrical pumping can be predicted fromthe threshold under optical pumping (see Materials and Methods forcalculation details). Thus, the observation of lasing under electricalpumping above thresholds of 0.6-0.8 kA cm⁻² for OSLD and OSLD-2 (whichhave the same grating periods as OSLD-6) is reasonable. Furthermore,slope efficiency (FIG. 9g-i ) was similar under optical and electricalpumping (0.4% and 0.3%, respectively), though it was markedly higher foran optically pumped device without electrodes (6%). These resultssuggest that the additional losses (including exciton annihilation,triplet and polaron absorption, quenching by the high electric field,and Joule heating) generally taking place in OLEDs at high currentdensities³⁴ have been nearly suppressed in the BSBCz OSLDs. This isfully consistent with the fact that EL efficiency rolloff was notobserved in the OSLDs under intense pulse electrical excitation. Thesuppression of losses can be explained based on the properties of BSBCzand the devices. As previously mentioned, BSBCz films do not showsignificant triplet losses (FIG. 10a )³⁵, and a decrease of the deviceactive area leads to a reduction of Joule-heat-assisted excitonquenching³⁶. The overlap between the polaron absorption and emissionspectrum is negligible for both radical cations and radical anions inBSBCz based on the measurement of composite films of BSBCz:MoO₃ andBSBCz:Cs, respectively (FIG. 10b ). Additionally, while metal losses area major problem in OLED structures, the DFB structure in the OSLDs ofthe invention reduces such losses by confining the light away from themetal.

Electrical and optical simulations of the devices were performed tofurther confirm that current-injection lasing is occurring in the OSLDs(FIG. 11). Using carrier mobilities extracted from the fitting ofexperimental data for unipolar devices (FIG. 11a, b ), simulated J-Vcurves for devices with and without a grating agreed well with theexperimental characteristics (FIG. 11 a, c, d), indicating sufficientetching for good electrical contact with ITO in the device with agrating. The recombination rate profile (FIG. 11e, f ) shows a periodicvariation inside the device because of periodic injection of electronsfrom the ITO electrode through the insulating SiO₂ grating. Similar tothe recombination, exciton density (S) is spread throughout thethickness of the organic layer (FIG. 12a ) but primarily concentrated inthe regions where SiO₂ does not hinder the path from cathode to anode.The average exciton density of the OSLD and OLED (FIG. 11g ) aresimilar, indicating that the high accumulation of excitons near the SiO₂compensates for the low exciton density between the grating (noinjection region) leading to a similar exciton density as for thereference device.

Light outcoupling from the second-order grating and light trapping inthe ITO layer, which forms a waveguide loss, are clearly visible in thesimulated electric field distribution E(x, y) of the optical field atthe calculated resonant wavelength 2₀=483 nm in the OSLD (FIG. 12b ).The DFB resonant cavity is characterized by a confinement factor F of40% and a quality factor of 255, which is consistent with a qualityfactor of 204 calculated from FIG. 7b using the equation λ_(peak)/FWHM.The modal gain (g_(m)), which is an indicator of the amplification oflight in the laser mode, as a function of current density was calculatedfrom the overlap of the exciton density distribution and optical fielddistribution (see Materials and Methods for details) with a stimulatedemission cross section σ_(stim) for BSBCz³⁵ of 2.8 10⁻¹⁶ cm⁻² and isshown in FIG. 12c for the second-order region. The high and increasingmodal gain above 500 A cm⁻² is consistent with the observation oflasing. The insulating DFB structure helps to enhance coupling with theoptical mode through localization of high exciton density in and abovethe valleys of the grating (FIG. 12a ), where the optical mode is strong(FIG. 12b ), resulting in the high values at J=500 A cm⁻² in FIG. 12 d.

In conclusion, the present invention proves that lasing from acurrent-driven organic semiconductor is possible through proper designand choice of the resonator and organic semiconductor to suppress lossesand enhance coupling. The lasing demonstrated here has been reproducedin multiple devices and fully characterized to exclude other phenomenathat could be mistaken for lasing. The results fully support the claimthat this is the first observation of electrically pumped lasing inorganic semiconductors. The low losses in BSBCz are integral to enablinglasing, so the development of strategies to design new laser moleculeswith similar or improved properties is an important next step. Thisreport opens new opportunities in organic photonics and serves as abasis for the future development of an organic semiconductor laser diodetechnology that is simple, cheap, and tunable and can enable fully anddirectly integrated organic-based optoelectronic platforms.

Materials and Methods

(Device Fabrication)

Indium tin oxide (ITO)-coated glass substrates (100-nm-thick ITO, AtsugiMicro Co.) were cleaned by ultrasonication using neutral detergent, purewater, acetone, and isopropanol followed by UV-ozone treatment. A100-nm-thick layer of SiO₂, which would become the DFB grating, wassputtered at 100° C. onto the ITO-coated glass substrates. The argonpressure during the sputtering was 0.66 Pa. The RF power was set at 100W. Substrates were again cleaned by ultrasonication using isopropanolfollowed by UV-ozone treatment. The SiO₂ surfaces were treated withhexamethyldisilazane (HMDS) by spin coating at 4,000 rpm for 15 s andannealed at 120° C. for 120 s. A resist layer with a thickness of around70 nm was spin-coated on the substrates at 4,000 rpm for 30 s from aZEP520A-7 solution (ZEON Co.) and baked at 180° C. for 240 s. Electronbeam lithography was performed to draw grating patterns on the resistlayer using a JBX-5500SC system (JEOL) with an optimized dose of 0.1 nCcm⁻². After the electron beam irradiation, the patterns were developedin a developer solution (ZED-N50, ZEON Co.) at room temperature. Thepatterned resist layer was used as an etching mask while the substratewas plasma etched with CHF₃ using an EIS-200ERT etching system(ELIONIX). To completely remove the resist layer from the substrate, thesubstrate was plasma-etched with O₂ using a FA-1EA etching system(SAMCO). The etching conditions were optimized to completely remove theSiO₂ from the grooves in the DFB until the ITO was exposed. The gratingsformed on the SiO₂ surfaces were observed with SEM (SU8000, Hitachi)(FIG. 1c ). EDX (at 6.0 kV, SU8000, Hitachi) analysis was performed toconfirm complete removal of SiO₂ from ditches in the DFB (FIG. 2c, d ).Cross section SEM and EDX were measured by Kobelco using acold-field-emission SEM (SU8200, Hitachi High-Technologies), an energydispersive X-ray spectrometry (XFlash FladQuad5060, Bruker), and afocused ion beam system (FB-2100, Hitachi High-Technologies) (FIG. 1d, e).

The DFB substrates were cleaned by conventional ultrasonication. Organiclayers and a metal electrode were then vacuum-deposited by thermalevaporation under a pressure of 1.5×10⁻⁴ Pa with a total evaporationrate of 0.1-0.2 nm s⁻¹ on the substrates to fabricate OSLDs with thestructure indium tin oxide (ITO) (100 nm)/20 wt % BSBCz:Cs (60 nm)/BSBCz(150 nm)/MoO₃ (10 nm)/Ag (10 nm)/Al (90 nm). The SiO₂ layers on the ITOsurface acted as insulators in addition to a DFB grating. Therefore, thecurrent flow regions of the OLEDs were limited to the DFB regions whereBSBCz is in direct contact with ITO. Reference OLEDs with an active areaof 30×45 μm were also prepared with same current flow region.

(Device Characterization)

All the devices were encapsulated in a nitrogen-filled glove box usingglass lids and UV-cured epoxy to prevent any degradation resulting frommoisture and oxygen. Current density-voltage-η_(EQE) (J-V-η_(EQE))characteristics (DC) of the OSLDs and OLEDs were measured using anintegrating sphere system (A10094, Hamamatsu Photonics) at roomtemperature. For pulse measurements, rectangular pulses with a pulsewidth of 400 ns, pulse period of 1 ms, repetition frequency of 1 kHz,and varying peak currents were applied to the devices at ambienttemperature using a pulse generator (NF, WF1945). Using theseconditions, the inventors could apply to a properly working OSLD from agood batch roughly 50 pulse at 1 kA cm⁻² (near threshold), 20 pulses at2 kA cm⁻², and 10 pulse at 3 kA cm⁻² before electrical breakdown.Approximately 500 devices were fabricated in this work with a yield ofabout 5%. The J-V-luminance characteristics under pulse driving weremeasured with an amplifier (NF, HSA4101) and a photomultiplier tube(PMT) (C9525-02, Hamamatsu Photonics). Both the PMT response and thedriving square wave signal were monitored on a multi-channeloscilloscope (Agilent Technologies, MSO6104A). The η_(EQE) wascalculated by dividing the number of photons, which was calculated fromthe PMT-response EL intensity with a correction factor, by the number ofinjected electrons, which was calculated from the current. Output powerwas measured using a laser power meter (OPHIR Optronics Solution Ltd.,StarLite 7Z01565).

To measure the spectra, emitted laser light for both optically andelectrically pumped OSLDs was collected normal to the device surfacewith an optical fiber connected to a multichannel spectrometer (PMA-50,Hamamatsu Photonics) and placed 3 cm away from the device. The beamprofile of the OSLDs was checked by using a CCD camera (beam profilerWimCamD-LCM, DataRay). For characteristics of OSLD-6 and OSL underoptical pumping, pulsed excitation light from a nitrogen-gas laser(NL100, N₂ laser, Stanford Research System) was focused in a 6×10⁻³ cm⁻²area of the device through a lens and slit. The excitation wavelengthwas 337 nm, pulse width was 3 ns, and repetition rate was 20 Hz. Theexcitation light was incident upon the devices at around 20° withrespect to the normal to the device plane. Excitation intensities werecontrolled using a set of neutral density filters. Steady-state PLspectroscopy was monitored using a spectrofluorometer (FP-6500, JASCO)in FIG. 10 and a spectrometer (PMA-50) in FIG. 6. Near-field patterns ofan OSL and an OSLD-6 were taken using a laser beam profiler (C9334-01,Hamamatsu Photonics) with near-field optics (A4859-06, HamamatsuPhotonics), and far-field patterns of an OSL were taken with the sameprofiler and near-field optics (A3267-11, Hamamatsu Photonics).

The lower limit for the electrical lasing threshold was determined fromthe optical threshold using the equation

${J = {2\frac{P_{th}{\lambda/{hc}^{\eta_{out}\phi_{PL}e}}}{\eta_{EQE}}}},$

where P_(th), λ, h, c, η_(out), φ_(PL), η_(EQE), and e are the opticalpumping threshold and wavelength, Planck constant, speed of light,device outcoupling efficiency, photoluminescence quantum yield of BSBCz,external quantum efficiency of a BSBCz OSLD, and elementary charge,respectively. This equation simply solves for the current density atwhich the rate of singlets generated under electrical excitation shouldbe equal to that under an optical excitation of P_(th). This equationdoes not account for additional loss mechanisms that occur underelectrical excitation at high current densities. The inventors used anη_(out) of 20% and φ_(PL) of 76% (from Table 2). Iterating over severalvalues in FIG. 3d to get good agreement between η_(EQE) and J, theinventors settled on a final η_(EQE) of 2.1%. The factor of two is toaccount for the fact that the inventors used only the exposed ITO area,which is half of the total grating area, when calculating currentdensities for OSLDs in this paper.

(Device Modeling and Parameters)

The optical simulation of the resonant DFB cavity was performed usingComsol Multiphysics 5.2a software. The Helmholtz equation was solved forevery frequency using the Finite Element Method (FEM) in the RadioFrequency module of Comsol software. Each layer was represented by itscomplex refractive index and thickness. The computation domain waslimited to one supercell composed of a second-order grating surroundedby first-order gratings. The Floquet periodic boundary conditions wereapplied for lateral boundaries, and scattering boundary conditions wereused for the top and bottom domains. Only TE modes were considered sinceTM modes are suppressed because they experience more losses than the TEmodes (due to metal absorption).

The charge transport through the OSLD was described using thetwo-dimensional time-independent drift-diffusion equation coupled to thePoisson equation and the continuity equation for charge carriers usingthe Technology Computer Aided Design (TCAD) software from Silvaco. Theelectron and hole concentrations were expressed using parabolic densityof states (DOS) and Maxwell-Boltzmann statistics. A Gaussiandistribution was used to model the trap distribution within the organicsemiconductor³⁷. The charge carrier mobilities were assumed to be fielddependent and have a Pool-Frenkel form^(38,39). In this model theenergetic disorder was not taken into account, so the inventors assumedthe validity of Einstein's relation to calculate the charge carrierdiffusion constant from the charge carrier mobility. The recombinationrate R was given by the Langevin model⁴⁰. The continuity equation forsinglet excitons is solved by taking into account the exciton diffusion,the radiative and non-radiative processes.

Experimental data for hole-only and electron-only devices (energydiagrams and structures in FIG. 4) were fitted to extract the chargecarrier mobilities. The values of the fitted mobility parameters andother input parameters used in the simulations are presented in Table 2.The extracted values were used to simulate bipolar OLED devices with thestructure ITO/20 wt % Cs:BSBCz (10 nm)/BSBCz (190 nm)/MoO₃ (10 nm)/Al.The work function of the cathode (ITO/20 wt % Cs:BSBCz) was taken to be2.6 eV and that of the anode (MoO₃/Al) 5.7 eV. The influence of the DFBgrating on the electrical properties of the OSLD were calculated andcompared to the reference device (without grating). The modal gain g_(m)was calculated from S(x, y) and the optical mode intensity |E(x, y)|²using the equation

${g_{m} = \frac{\sigma_{stim}{\int_{0}^{L}{\int_{0}^{d}{{{E\left( {x,y} \right)}}^{2}{S\left( {x,y} \right)}{dydx}}}}}{\int_{0}^{L}{\int_{0}^{d}{{{E\left( {x,y} \right)}}^{2}{dydx}}}}},$

where L is cavity length (only the second-order grating region) and d isthe active film thickness.

Near- and far-field patterns were simulated using the OptiFDTD softwarepackage (Optiwave). Near-field patterns were simulated using the FDTDmethod. From these patterns, the Fraunhofer approximation was used tocalculate the far-field pattern. Perfect matched layer and periodicconditions were used as the boundary conditions.

TABLE 2 Parameters for optical and electrical simulations. ParameterBSBCz BSBCz:Cs Units ε_(r) 4 4 — E_(HOMO) 5.8 5.8 eV E_(LUMO) 3.1 3.1 eVN_(HOMO) 2 × 10⁻¹⁹ 2 × 10⁻¹⁹ cm⁻³ N_(LUMO) 2 × 10⁻¹⁹ 2 × 10⁻¹⁹ cm⁻³N_(tp) 2.8 × 10⁻¹⁷   — cm⁻³ E_(tp) 0.375 — eV σ_(tp) 0.017 — eV μ_(n0)6.55 × 10⁻⁵    6.55 × 10⁻⁵    cm² V⁻¹ s⁻¹ μ_(p0) 1.9 × 10⁻⁴    1.9 ×10⁻⁴    cm² V⁻¹ s⁻¹ F_(n0) 175,561 175,561 V cm⁻¹ F_(p0) 283,024 283,024V cm⁻¹ k_(r) 0.6 × 10⁹    0.6 × 10⁹    s⁻¹ k_(nr) 0.18 × 10⁹    0.89 ×10⁹    s⁻¹ φ_(PL) 0.76 0.4 — L_(S) 18 18 nmε_(r) is the relative permittivity of the material. E_(HOMO) andE_(LUMO) are the energy levels of the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively.N_(HOMO) and N_(LUMO) are the densities of states of the HOMO and theLUMO levels. N_(tp) is the total density of trap, E_(tp) is the energydepth of traps above the HOMO level and σ_(tp) is the width of theGaussian distribution. μ_(n0) and μ_(p0) are the zero-field mobility.F_(n0) and F_(p0) are the characteristic electric field for electron andhole, respectively. k_(r) is the radiative decay constant and k_(nr) isthe non-radiative decay constant. φ_(PL) is the photoluminescencequantum yield. L_(S) is the exciton diffusion length. As anapproximation, the mobilities of BSBCz:Cs were set to be the same asthose of BSBCz, which resulted in good fits to experimental data, so themobilities were not further refined.

1. A current-injection organic semiconductor laser diode comprising apair of electrodes, an optical resonator structure, and one or moreorganic layers including a light amplification layer composed of anorganic semiconductor, which has a sufficient overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection toemit laser light.
 2. The current-injection organic semiconductor laserdiode according to claim 1, wherein the optical resonator structure hasa distributed feedback (DFB) structure.
 3. The current-injection organicsemiconductor laser diode according to claim 2, wherein the opticalresonator structure is composed of a second-order Bragg scatteringregion surrounded by a first-order Bragg scattering region.
 4. Thecurrent-injection organic semiconductor laser diode according to claim2, wherein the second-order Bragg scattering region and the first-orderBragg scattering region are formed alternately in the optical resonatorstructure.
 5. The current-injection organic semiconductor laser diodeaccording to claim 1, wherein the number of the one or more organiclayers is 2 or less.
 6. The current-injection organic semiconductorlaser diode according to claim 1, wherein the thickness of the lightamplification layer relative to the total thickness of the one or moreorganic layers is more than 50%.
 7. The current-injection organicsemiconductor laser diode according to claim 1, wherein the organicsemiconductor contained in the light amplification layer is amorphous.8. The current-injection organic semiconductor laser diode according toclaim 1, wherein the molecular weight of the organic semiconductorcontained in the light amplification layer is 1000 or less.
 9. Thecurrent-injection organic semiconductor laser diode according to claim1, wherein the organic semiconductor contained in the lightamplification layer is a non-polymer.
 10. The current-injection organicsemiconductor laser diode according to claim 1, wherein the organicsemiconductor contained in the light amplification layer has at leastone stilbene unit.
 11. The current-injection organic semiconductor laserdiode according to claim 1, wherein the organic semiconductor containedin the light amplification layer has at least one carbazole unit. 12.The current-injection organic semiconductor laser diode according toclaim 1, wherein the organic semiconductor contained in the lightamplification layer is 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz).13. The current-injection organic semiconductor laser diode according toclaim 1, which has an electron injection layer as one of the organiclayers.
 14. The current-injection organic semiconductor laser diodeaccording to claim 13, wherein the electron injection layer contains Cs.15. The current-injection organic semiconductor laser diode according toclaim 1, which has a hole injection layer as an inorganic layer.
 16. Thecurrent-injection organic semiconductor laser diode according to claim15, wherein the hole injection layer contains molybdenum oxide.
 17. Amethod for designing a current-injection organic semiconductor laserdiode, comprising: selecting materials and designing structure of thediode so as to increase the overlap between the distribution of excitondensity and the electric field intensity distribution of the resonantoptical mode during current injection.
 18. A method for producing acurrent-injection organic semiconductor laser diode, comprising:evaluating the overlap between the exciton density distribution and theelectric field intensity distribution of the resonant optical modeduring current injection in a designed or existing diode, changing atleast one of the materials and the structure of the diode to so design anew diode as to increase the overlap between the distribution of excitondensity and the electric field intensity distribution of the resonantoptical mode during current injection, and producing the new diode. 19.A current-injection organic semiconductor laser diode produced by themethod of claim
 18. 20. A program for designing current-injectionorganic semiconductor laser diode, which designs a current-injectionorganic semiconductor laser diode to increase the overlap between thedistribution of exciton density and the electric field intensitydistribution of the resonant optical mode during current injection.